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Abstract

We have fabricated nanocomposites consisting of 4-Å carbon nanotubes embedded in the 0.7-nm pores of aluminophosphate-five (AFI) zeolite that display a superconducting specific heat transition at 15 K. MicroRaman spectra of the samples show strong and spatially uniform radial breathing mode (RBM) signals at 510 cm−1 and 550 cm−1, characteristic of the (4, 2) and (5, 0) nanotubes, respectively. The specific heat transition is suppressed at >2 T, with a temperature dependence characteristic of finite-size effects. Comparison with theory shows the behavior to be consistent with that of a type II BCS superconductor, characterized by a coherence length of 14 ± 2 nm and a magnetic penetration length of 1.5 ± 0.7 μm. Four probe and differential resistance measurements have also indicated a superconducting transition initiating at 15 K, but the magnetoresistance data indicate the superconducting network to be inhomogeneous, with a component being susceptible to magnetic fields below 3 T and other parts capable of withstanding a magnetic field of 5 T or beyond.

↵3Present address: Research and Development, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia.

The authors declare no conflict of interest.

*

↵* The total C/T measured at 15 K is 5.5× 10−6 J/K2 (corrected for GE varnish), and Celectr/T = 1.5× 10−8 J/K2 represents the low temperature difference between the 5-T data and the zero field data. This value was taken to be γn. The numbers in the text were obtained from the following useful data. AFI zeolite density is 1.87 g/cm3 and the (25,0) carbon nanotube's unit cell length is 4.32 Å, with 20 carbon atoms to each unit cell. Per unit length of the pore [filled with (5,0) carbon nanotubes], the mass of the carbon is 32% that of AFI zeolite, which has the composition of Al12P12O48, with a unit cell length of 8.4 Å. In Fig. 2, the mass used in the Celectr/T normalization was mostly that of AFI zeolite.

†

↵† For example, if it is known that the average physical size of the nanotube clusters is 5 times as long (along the c-axis) as it is wide (i.e., 80 nm wide and 400 nm in length), then from our simulation results it can be concluded that the coherence length in the z direction is 10 × ξ0, which implies a factor of 100 in the ratio of the coupling strength along the transverse and longitudinal directions.

‡

↵‡ Differential resistance measurement was done by using a negative, 2-nA and 0.02-s current pulse followed by a similar positive pulse, and measuring the voltage difference. This is repeated for every current value.

§

↵§ We note that in spite of the different samples and their fabrication processes, there is consistency in the measured transition temperature. This could be due to the ordered AFI crystal structure. Also, it should be noted that the constancy of the 15 K transition temperature greatly decreases the possibility that the superconductivity is due to unintentional nitrogen doping, since in the current sample fabrication approach the nanotubes are formed from ethylene (instead of TPA as in the original approach), which has no nitrogen. If the superconductivity was due to nitrogen doping, then at least the transition temperature should be sensitive to the doping level. The different fabrication approaches should, at a minimum, alter the nitrogen doping level. Thus, the constancy of the transition temperature may be regarded as strong evidence against nitrogen doping as the cause of the observed superconductivity. The lack of magnetic anisotropy also represents a significant difference with the previous samples. This could be due to the larger transverse dimension of the nanotube clusters in the present samples.

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